The invention relates to a drive control system for a sewing machine. More specifically, it relates to a system for stopping automatically a sewing machine when a thread of the machine is tangled or broken.
Most common industrial and household sewing machines in use today were equipped with drive control systems having a foot controller. The foot controller comprises a switch contact and a variable resistor having a resistor and a slider. The slider normally touches the switch contact and moves along the resistor in correspondence with downward force on the foot controller by an operator. The sewing machine is at standstill when the slider touches the switch contact and begins to move when the slider touches an open terminal of the resistor by the downward force on the foot controller. The sewing speed is accelerated with the advance of the movement of the slider toward the other terminal of the resistor by increasing the downward force. Decreasing the downward force, the slider moves toward the switch contact and the sewing speed is decelerated, then the sewing machine stops when the slider touches the switch contact. Therefore, it takes a relatively long time to stop the sewing machine after an accidental skip of sewing stitches, a tangle of a cloth on the sewing machine, or a break, consumption or tangle of an upper or lower sewing thread. Because the detection of the accidents by the operator is sometimes tardy the release of the downward force from the foot controller is delayed after the accident. However, it is desirable to stop the sewing machine as fast as possible after an occurrence of the accidents of the sewing threads or the cloth.